Complex polynomials

Show that if the polynomial p(z) = anzn+an-1zn-1+…+a0 is written in factored form as p(z) = an(z-z1)d1(z-z2)d2…(z-zr)dr, then
(a) n = d1+d2+…+dr
(b) an-1 = -an(d1z1+d2z2+…drzr)
(c) a0 = an(-1)nz1d1z2d2…zrdr